Method and apparatus for compressed mode communication

ABSTRACT

A method of compressed mode communications permits evaluation of one communication system while communicating in another communication system. User equipment devices ( 108, 110 ) are assigned to different portions of a frame during compressed mode.

FIELD OF THE INVENTION

The present invention pertains to framed signaling, and moreparticularly to a method and apparatus utilizing compressed modeoperation for framed signals.

BACKGROUND OF THE INVENTION

Third generation wireless mobile user equipment will support dual radioaccess technology, such as by supporting communication over 3G (thirdgeneration) systems, such as wideband code division multiple access(WCDMA) systems, and 2G systems, such as Global Systems for Mobilecommunications (GSM) systems. Such user equipment will be required toacquire and maintain knowledge of multiple radio frequency domains withregard to signal strength of serving and adjacent cells, interference,and synchronization. When such user equipment is operating in idle mode,which is the mode where the user equipment is not engaged in dedicatedcommunication with a serving cell, the implementation of such proceduresis straightforward.

However, where the user equipment is engaged in dedicated communicationon a serving cell of one system, requiring that it both receive andtransmit signals, there may be a lack of time available during whichmeasurements or synchronization of the other systems supported by theequipment can take place. For example, if user equipment is engaged indedicated communication with a serving cell on the Universal TerrestrialRadio Access (UTRA) domain using frequency division duplex (FDD), theuser equipment must transmit during each available frame period. Thislimits the time available for performing measuring and synchronizationwith a cell of a GSM system.

To overcome this problem, third generation partnership project (3GPP)specification section 25.212 specifies “compressed mode” operation,during which the mobile user equipment, or the network, may transmitduring only a portion of a frame in order to allow measurement and/orsynchronization during the other portion of the frame. However, thisspecification requires transmissions to be performed using a smallerspreading factor, thereby necessitating a 3 dB greater transmissionpower to achieve a suitable bit-error rate (BER). The specified methodthus severely impacts the capacity of the cell, as the number of devicesoperating in compressed mode will be limited by the increased powerrequirements.

The 3GPP specification describes three methods for reducing the signallength to create a transmission gap for compressed mode operation.Puncturing, by which data redundancy is removed for a compressed frameto allow transmission within a shorter time period. This techniqueallows more data to be transmitted at the expense of error correctioncapability. A second technique is spreading factor reduction, by whichthe spreading factor is reduced by a factor of 2, thereby requiring halfthe time to transmit a given amount of data. However, such a reductionis at the expense of processing gain, which is applicable to both theuplink and the downlink. A third method of reducing the signal length ishigher layer scheduling.

What is needed is an improved compressed mode communication.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects, features and advantages of the present inventionwill become more fully apparent from the following Detailed Descriptionwith the accompanying drawings described below.

FIG. 1 is schematic representation of a cellular communication systemhaving different overlapping systems.

FIG. 2 is a circuit schematic in block diagram form illustrating userequipment and four base stations.

FIG. 3 illustrates a compressed mode frame allocation.

FIG. 4 illustrates an improved compressed mode frame allocation.

FIG. 5 illustrates user equipment operable in compressed mode.

FIG. 6 illustrates a base station operable in compressed mode.

FIG. 7 illustrates signal flow between the user equipment and the basestation.

FIG. 8 illustrates another improved compressed mode frame allocation.

FIG. 9 is a flow diagram illustrating operation of a base station toassign portions of a frame for compressed mode communication.

FIG. 10 is a flow diagram illustrating an alternate operation of a basestation to assign portions of a frame for compressed mode communication.

FIG. 11 is a flow diagram illustrating operation of user equipment andbase station to allocate the portion of the frame for compressed modecommunication.

FIG. 12 is a flow diagram illustrating an alternate operation of a userdevice and a base to allocate the portion of the frame for compressedmode communication.

FIG. 13 illustrates another embodiment of a compressed mode frameallocation.

FIG. 14 illustrates a compressed mode pattern.

DETAILED DESCRIPTION OF THE DRAWINGS

A complementary compressed mode method and apparatus facilitateevaluation of one communication system while communicating in anothercommunication system. User equipment devices (108, 110) are assigned todifferent portions of a frame during compressed mode operation.

A cellular communication system 100 (FIG. 1) is illustrated including afirst communication system 102 covering a plurality of cells 101 (onlysome of which are numbered). By way of example, each cell can beconsidered to represent the coverage area of a base station. The firstsystem may for example be a UTRA system, and in particular either a UTRAFDD or UTRA time division duplex (TDD) system. The communication system100 also includes a second communication system 104 covering a pluralityof cells 103 (only some of which are numbered), each cell representingthe coverage are of a base station. The second system may for example beGlobal System for Mobile communications (GSM) or a second generation(2G) code division multiple access (CDMA) system. The cellularcommunication system may include additional or other communicationsystems, as the communication systems may operate according to any knownwireless communication system specification such as the GSM, CDMA,wideband code division multiple access (WCDMA), time division multipleaccess (TDMA), Group Packet Radio System (GPRS), EDGE, or the like. Theuser equipment 108, or user equipment device, may be a cellularradiotelephone, a personal digital assistant, a modem, an accessory, orthe like, and may support either single mode or multimode operation, andthus may be capable of operating in one or more than one communicationprotocol and/or one or more than one frequency band.

The user equipment 108 includes transceiver 204 (FIG. 2) and acontroller 205. In general, the transceiver 204 enables the userequipment to effect a wireless communication link with base stations,such as base station 200 of cell 101′, base station 230 of cell 101″,base station 210 of cell 103″, and base station 220 of cell 103′. Eachbase station includes a transceiver 206, 212, 222, 232, for wirelesscommunication and a controller 208, 214, 224, 234 for controlling theoperation of the base station and establishing a communication link withthe mobile switching center 240. The mobile base stations (e.g., 200,220, 220, 230) and the mobile switching center 240 are a networksupporting wireless communications.

In operation, as the user equipment 108, 110 moves through the system,hand-off will occur according to ordinary operating techniques, whichare well known in the art. For multi-mode user equipment, such as thoseoperating over a plurality of different communication air interfaces,the user equipment 108, 110 will be required to acquire and maintainknowledge of multiple radio frequency domains, and may for examplemaintain knowledge of signal strength of serving and adjacent cells,interference information, and synchronization information, as is knownto those of ordinary skill in the art.

While the user equipment 108, 110 establishes a dedicated communicationlink with a base in the first communication system 101, which isillustrated as a UTRA system, the user equipment will at leastoccasionally be required to evaluate system 104, a GSM system. This mayfor example occur when the user equipment 108 moves to the edge of cell101′, 101″ adjacent cell 103′. In the illustrated embodiment, cell 103′covers an area not served by communication system 102, and thus userequipment 108 will need to be handed off from base station 200 to basestation 220. In order to support the measurement and synchronizationprocesses that user equipment 108 must perform while engaged incommunications with base station 200 of cell 101′, at least the uplinkcommunications between the user equipment 108 and base station 200 aremade in compressed mode.

More particularly, the user equipment 108 (FIG. 1) will be required toobtain knowledge of multiple radio frequency domains, and attend tointer-domain measurement and/or synchronization tasks for the secondcommunication system 104 while has an established link withcommunication system 102. In compressed mode, these tasks are performedduring a time period when the user equipment 108 is not in dedicatedcommunications with communications system 102. One timing diagram foraccomplishing this is illustrated in FIG. 3, showing timing for userequipment 108, 110, referred to in this figure as Mobile A (108) andMobile B (110). In compressed mode Mobile A and Mobile B both transmitwith a reduced spreading factor, and higher power, such thatcommunications with one system (e.g., base station 200) occur in thefirst half of the frame. During the second half of the frame, Mobile Aand Mobile B may evaluate the other system. Evaluation may for examplecomprise making inter domain measurements, obtaining synchronizationwith the other system, or another system evaluation (e.g., base station220).

Thus, in compressed mode, a transmission gap is created during which theuser equipment may perform measurements without encountering ascheduling conflict. A scheduling conflict would otherwise occur wherethe user equipment attempts to perform two tasks simultaneously with asingle transceiver path. Additionally, compressed mode occurs withoutsubjecting the system to prohibitively high levels of self-interference,as in the case of inter-mode measurements that may occur at the sametime in the same or a close frequency band.

In normal mode, the CDMA signals from user equipment 108, 110 areseparated from one another by a channel identification code on theuplink. The signals in the downlink are also separated by a channelidentification code. During compressed mode, the channel identificationcode (e.g., an orthogonal code in CDMA) still isolates the signals, butthe information rate is effectively “sped up” by a factor of 2 inresponse to the spreading factor being reduced by ½. It is necessary toincrease uplink power for the user equipment 108, 110 in compressed modeto compensate for the loss of processing gain due to the lower spreadingfactor. A significant problem encountered with a system operatingaccording to FIG. 3 is the number of user equipment devices that canoperate in compressed mode is severely limited.

As used herein, in a “compressed mode pattern,” a certain number offrames having transmission gaps are followed by a certain number offrames that do not have transmission gaps, and this pattern repeats witha periodicity of a certain number of frames. Compressed mode patternthus refers to: the number of time slots during which transmissionoccurs within the period of a given frame; the number of time slotsduring which compression does not occur within the period of a givenframe; the number of compressed frames in which compressed transmissionsoccur during time slots of a given frame; and the number ofnon-compressed frames in which compressed transmission does not occurduring the time slots of a given frame.

An example of a transmission pattern is illustrated in FIG. 14. Theillustrated pattern comprises 12 frames, 2 compressed frames followed by10 non-compressed frames. Within the compressed frames, there are 15slots, the first 4 and last 4 of which are available for transmissionand the middle 7 of which the transmitter is turned off. Those skilledin the art will recognize that many other transmission patterns arepossible.

A significantly improved system for compressed mode operation isillustrated in FIG. 4. In FIG. 4, Mobile A (108 in FIG. 1) communicatesin the first portion of the frame and Mobile B (110 in FIG. 1)communicates in the second portion of the frame. While Mobile Acommunicates with one base station 200 of cell 101′ of the firstcommunication system 102, Mobile B performs inter-domain measurementand/or synchronization with base station 220 of cell 103′. In the secondportion of the frame, Mobile B communicates with a base station 200 andMobile A performs inter-domain measurement and/or synchronization. Thecompressed mode continues for N frames, where N is an integer. N can beany number greater than 0, and may for example be 2, such thatmeasurements may be made in 2 consecutive frames followed by 10 framesthat are not compressed, thereby providing a 12-frame pattern. Although2 user equipment devices are illustrated, more than 2 user equipmentdevices can be allocated to each of the first portion and the secondportion of the frame, each of the user equipment devices having arespective orthogonal code, and because all of the user equipment is notcommunicating within the same portion of the frame, the number ofdevices that can operate in compressed mode is significantly increased.

Although the first portion and the second portion may be allocated frommany different groups of slots, one pattern envisioned is to divide theframe into 15 slots. The first portion comprises 7 slots that areallocated to a plurality of devices separated by orthogonal codes. Thesecond portion comprises the last 7 slots that are allocated to anothergroup of devices, also separated by orthogonal codes. The third portionis a separation slot in the middle of a frame. In one embodiment, it isenvisioned that the devices allocated to the first portion will havedifferent orthogonal codes than the devices in the second portion. Theslots are preferably of equal length.

To examine the effects of amplitude variations based on the type ofpattern (on/off sequence) selected, a simulation was used to generatevarious compressed mode patterns in terms of the radio frequency (RF)envelope shape, and then a Fourier transform was used to determine thespectral properties of the envelope. The 7-1-7 slot compressed modepattern was found to have favorable spectral characteristics whencompared to other patterns when user equipment were paired.

In particular, Fourier analysis was used to compute the spectrum of theRF envelope having the maximum allowable rise time and a decay time of25 μs. The following simulation used compressed mode patterns, each ofwhich has a repetition period of 12 frames, i.e. 2 compressed framesfollowed by 10 uncompressed frames, which pattern was repeated. Theinventors found a significant degree of cancellation of spectralcomponents under 100 Hz (frequency number approximately 125) for the7-1-7 pattern. While there are many other combinations of patterns thatmay be compared and utilized with the invention, the 7-1-7 pattern usinga repetition period of 12 frames resulted in lower uplink interference.

The user equipment 108 will now be described in greater detail withreference to FIG. 5. The user equipment includes transceiver 204, whichmay be implemented using any suitable wireless transceiver known in theart. The controller 205 includes a physical layer 504, a medium accesscontrol (MAC) layer 506, a radio link controller (RLC) layer 508, and aradio resource control layer (RRC). The physical layer 504 maps thetransport channels to physical channels and exchanges coded andmodulated baseband signals with the RF transceiver. The channels may beidentified by frequency, code (such as in a CDMA system) or time (suchas in a TDMA system), or by any two or more of frequency, time and code.

The MAC layer 506 maps logical channels from the RLC 508 to transportchannels in the physical layer. The RLC 508 controls the transmissionlink over the radio medium.

The RRC layer 510 controls radio operation of the user equipment 208 (or210). RRC layer 510 includes a control message recognizer 514, whichoutputs downlink messages to the control message parser 516. Compressedmode control messages are input to the uplink compressed mode controller518. The compressed mode controller generates message acknowledgements,which are input to the uplink user data path for communication to thebase station. The uplink compressed mode controller also generatescompressed mode control information, pattern assignment information, andresource assignment and measurement scheduling, which is determined asdescribed in greater detail hereinbelow. The physical layer includes anuplink compressed mode pattern and assignment manager 520 responsive tothe compressed mode pattern received from the uplink compressed modecontroller 518. An uplink transmission controller 522 communicates viathe radio frequency transceiver 204 under the control of resourceassignments received from the uplink compressed mode controller 518. Thephysical layer further includes a measurement acquisition unit 524,responsive to the measurement schedule from the uplink compressed modecontroller 518 for acquiring measurements and communicating themeasurements to the measurement processor 526 in the radio resourcecontroller 510.

FIG. 6 illustrates the base stations in cellular communication system100, and is represented herein be base station 200 (210, 220, 230). Thebase station 200 includes RF transceiver 206. The controller 208includes a physical layer 604, MAC 606, RLC layer 608, and RCC layer610. The physical layer maps transport channels to basic physicalchannels, and generates coded and modulated baseband signals. The MAClayer maps logical channels to transport channels. The RLC layercontrols radio bearers or transmission links over the radio medium.

The RCC layer 610 controls radio resources. The RCC includes an uplinkcompressed mode controller 614, which receives downlink signal data andgenerates uplink signal data. The compressed mode controllercommunicates with the uplink traffic scheduler 616. Additionally, theuplink compressed mode controller communicates the measurement scheduleto the measurement acquisition unit 612. A measurement processor 618receives the measurements from the measurement acquisition unit 612.

The operation of the system will now be described with reference to FIG.7. Initially, the user equipment devices 208, 210, (represented byMobile A and Mobile B) send a message containing measurement capabilityinformation element to the network (base station 200). The informationelement may be part of multiple messages, and contain the need, purposeand direction for the type of compressed mode requested. When the RRClayer 610 is making an assignment of radio resources to user equipmentbeing scheduled for uplink transmission, the uplink compressed modecontroller 614 in the network RRC 610 chooses pairs of user equipmentdevices and assigns them to complementary patterns and the same startingframe number. The characteristics of the uplink compressed mode are sentin an assignment message containing the compressed mode information IEwhich includes: compressed mode pattern; starting frame number; startingtimeslot within frame; pattern period; and the maximum number ofrepetitions. In the example of FIG. 4, Mobile A will be assigned totransmit in timeslots TS0-TS6 and Mobile B will be assigned to timeslotsTS8-TS14. Thereafter, Mobile A and Mobile B transmit uplink blocks usingthe assigned communication slots in the current communication domain(e.g., communication system 102) and measure and/or synchronize on theother communication domain (e.g., communication system 104). Theprocedure repeats for the duration of the frames indicated.

FIG. 9 illustrates operation of the base station of system 102 to assigntime slots according to one embodiment. In particular, as user equipmentis added to compressed mode, the network first determines how many userequipment devices are operating in compressed mode and transmitting inportion 1 of the frame (time slots TS0-TS6) for frames 1 and 2, asindicated in step 902. The network then determines how many userequipment devices are operating in compressed mode and communicating thesecond portion of the frame (time slots TS8-TS14) for frames 1 and 2, instep 904. The newest addition to compressed mode operation in frames 1and 2 is then assigned by the network to the time slots having thefewest number of user equipment devices operating in compressed mode, instep 906. The assignment is stored in the network and communicated tothe user equipment in step 908.

An alternate embodiment is illustrated in FIG. 10. In this embodiment,when the subscriber equipment initiates compressed mode operation, thenetwork determines in step 1002 to which portion (e.g., time slotsTS0-TS6 or time slots TS8-TS14) the user equipment that last initiatedcompressed mode operation in the same frames was assigned in step 1004.The network assigns the new user equipment device to the other portionof the frames, in step 1006.

Yet another alternate embodiment for assigning compressed mode operationis illustrated in FIG. 11. In FIG. 11, both the network and the userequipment operation is described, as in this embodiment the assignmentis not made at the network, but rather the user equipment and the basestation both determine the compressed mode slots for the user equipmentusing a deterministic value known to both the user equipment device andthe base station. Thus, in this embodiment, a predetermineddeterministic value known to both the user equipment and the networkselects the portion of the frame to which the user equipment isassigned. In particular, compressed mode operation is initiated in step1102. The deterministic value is checked in step 1104. If thedeterministic value is a 1, the user device will conduct compressed modeoperation in portion 1 of the time slot, as indicated in step 1108. If1, as determined in step 1106, the compressed mode communication willtake place in the second portion of the frame, and measurements will bemade in the first portion of the frame.

It is envisioned that the deterministic value may be any value known tothe user equipment and the base station, and may for example be aparticular bit of the subscriber equipment IMEI, such as the last bit ofthe subscriber equipment IMEI. An alternative deterministic value couldbe a predetermined bit of a signal communicated from the user equipmentto the network, such as a bit stored in memory in the user equipment.Another alternative can be a random or pseudo-random number generated bycircuitry in the user equipment and known to base station.

FIG. 8 illustrates an alternate frame allocation embodiment. In theembodiment of FIG. 8, the mobile user equipment slot assignments varyfrom frame to frame. Thus, in frame 2 (F2), Mobile A is transmits in thefirst portion and Mobile B transmits in the second portion. In frame 3(F3), Mobile B transmits in the first portion and Mobile A transmits inthe second portion. In the illustrated example, Mobile A and Mobile Btransmit in different portions of the frame. Those skilled in the artwill recognize that the assignments for each of Mobile A and Mobile Bmay advantageously be random, or pseudo random, such that in some framesMobile A and Mobile B will communicate on one system and measure in theother system during the same time slots of a frame. Those skilled in theart will also recognize that there will be more than 2 user equipmentdevices operating on the system, such that many other mobiles aretransmitting in each of the first and second portions of the frame. Itis envision that the compressed modes illustrated in this applicationcan apply to systems operating at capacity, with all the user equipmentdevices operating in compressed mode.

FIG. 12 illustrates operation of the user equipment and base station,wherein each use a deterministic value to determine the frame portionfor communication such that the base station need not determine andassign the value to the subscriber device, and providing the compressedmode frame allocation shown in FIG. 8. In FIG. 12, the portionassignment for communication with base station 200 will change in eachframe in a pseudo-random manner. The compressed mode is initialized instep 1202. The deterministic value for the first frame is determined instep 1204. For example, the deterministic value can be based on theencryption sequence that is generated for encrypting data in digitalcommunication systems, which is a pseudo random number known to the userequipment device and base station, can be used as the basis for makingthe slot assignment for a user equipment device in a frame. Anotheralternative is for the user equipment device and the network to each usea predetermined bit of a synchronized linear shift register that stepsthrough every number (or every number except all zeroes) as thedeterministic value. For example, the last bit of the sequence selectedto be the basis for the slot assignment can be used as the deterministicvalue.

In step 1206, the controller in the user equipment and the base stationdetermines whether the deterministic bit is a 1 or a 0. If the bit is a0, then the communication in compressed mode will be in portion 1 forthe initial frame. If the bit is a 1, the communications in thecompressed mode will use portion 2 for the initial frame, as indicatedin step 1208. The controller waits for the next frame in step 1214. Ifthe next frame is after the last frame of the compressed mode sequence,as determined in step 1216, the compressed mode communication ends. Ifthe frame is not after the last frame of the compressed mode sequence,the controller identifies the deterministic value for the next frame instep 1218, and returns to decision step 1206. This process will berepeated for the compressed frames in the compression pattern.

With complementary compressed mode, the uplinks are still processed byreducing their spreading factor by ½, except that instead of beingisolated from one another by orthogonal codes, they are now temporarilyisolated in a manner of a slotted physical access mechanism.Additionally, compressed mode operation can be assigned to pairs of userequipment devices, each with a complementary pattern.

The present invention has significant technically and commerciallydesirable attributes. It reduces the peak-to-peak amplitude variationsarriving at the Node-B receiver from a given pair of user equipmentsassigned to symmetrical compressed mode. This results in lowerself-interference on the uplink, and therefore greater cell capacity.Additionally, a timeslot pattern and period can be selected thatdemonstrates surprising spectral characteristics for the RF envelope tobe optimized.

FIG. 13 illustrates a system wherein radio resource assignments are madefor a universe of 4 user equipment devices that would previously havebeen allocated to 2. This is accomplished by assigning the same channelidentification code, such as the spreading or orthogonal code of a CDMAsystem, to Mobile B and Mobile C, and another channel identificationcode to Mobile A and Mobile D. The user equipment operates in compressedmode to increase the capacity of the system, by assigning the same codeto a pair of user equipment devices that operate within differentportions of the frame. This assignment can be utilized in those frameswhere compressed mode is used as described herein above, and in systemswhere compressed mode is not utilized for measurement andsynchronization activity on other domains. In both cases, the compressedmode is used to increase capacity for best-effort packet transmission byan amount approaching 66%. The 66% improvement is dictated by therequirement that 1 of 3 frames must be transmitted in a non-compressedmanner. Thus, an intelligent algorithm permits user equipment devices toachieve a substantially higher capacity for best-effort packet transfermode as more user equipment devices may be assigned to the same code andmultiplexed in time as well as by code. It is expected to be especiallyuseful in the case of a best effort packet data transmission where anintelligent complementary compressed mode scheduler would produce pairsof radio resource assignments and compressed mode patterns based on theradio resource availability as well as uplink signal quality. In thismanner, multiple user equipment may share the same codes on the uplink.

While the present inventions have been described in a manner thatenables those of ordinary skill in the art to make and use theinventions, it will be understood and appreciated that there are manyequivalents to the exemplary embodiments disclosed herein and thatmodifications and variations may be made without departing from thescope and spirit of the inventions, which are to be limited not by theexemplary embodiments but by the appended claims.

1. A method of operating a user equipment to measure and synchronizewith one radio communication system while communicating with an otherradio communication system, at least the other communication systembeing a framed communication system, the method comprising the steps of:communicating with the other communication system in a first portion ofa frame; evaluating the first communication system during a secondportion of the frame; and changing the portion of the frame where thecommunicating and evaluating occur over a sequence of compressed frames.2. (canceled)
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